To the left is an image of the youngest section of a typical root. It can be divided into several regions. At the very tip is the root cap and the apical meristem where new root cells are produced by cell division. These cells grow larger in the region of elongation and start to specialise in their function in the region of maturation. It is in the region of maturation that the root hairs are found that are so important to the uptake of nutrients and water for the plant by the root.
Below is a longitudinal section through a root taken right at the tip. It is here that the cells which will form the new root are produce by the division of the meristematic cells (dividing cells) of the apical meristem.
|A diagram of
a longitudinal section through
the tip of a root.
At the centre of the apical meristem is a quiescent centre which consists of approximately 500 to 1000 cells that do not appear to be very active. Researchers believe that these cells may serve as a reservoir for the other cells of the meristem should these be damaged.
Cells towards the top of the apical meristem divide, forming new cells that enter the region of elongation where they increase in size. These cells are destined to become the cells which make up the young parts of the root. The cells on the lower part of the apical meristem divide to form a structure which is a specific characteristic of roots (it is not present in stem) - the root cap.
The root cap
The cells which are formed by the lower portion of the apical meristem differentiate or change into cells which are arranged in columns and are therefore known as columella cells. The columella cells contain amyloplasts which tend to sink to the bottom of each cell. Roots may be able to sense gravity and therefore also ways grow downwards due to this characteristic of the columella cells. After 2-3 days the columella cells are pushed towards the outside of the root cap by the new cells which are being formed. Here they differentiate into periferal cells which are eventually sloughed off the root cap by the action to the root growing between the rough soil particles. The periferal cells secrete a slimy mucous-like substance known as mucilage.
Functions of the root cap
Together, the root cap and the mucilage it secretes, have a number of important functions:
1. They protect the root. The root cap physically protects the delicate dividing tissues from the abrasive action of the root moving between the rough soil particles as it grows. The mucilage protects the root from becoming dried out.
2. The mucilage lubricates the root for movement between the soil particles
3. The water-absorbing properties of the mucilage helps to increase the root's ability to absorb water.
4. The chemical composition of the mucilage helps the uptake of nutrient ions in the surrounding soil by the root.
Behind the apical meristem of the root is an area known as the region of elongation,
which is about 4-10mm in length. It is in this zone that the root cells begin to elongate
primarily by filling their cell
vacuoles with water. These cells are thus easily
distinguished from the cells of the apical meristem by the fact that they are elongated
with large water filled vacuoles, whereas the apical meristem cells are small and densely
filled with cytoplasm. It is this action of cellular elongation which pushes the root tip with the
root cap forward, through and between the soil particles.
Note: growth in the root and any plant structure is due to a combination of the increase in the number of cells which takes place in the apical meristem in roots, and also in the size of the individual cells which takes place in the region of elongation in the root.
Behind the region of elongation is a region of differentiation. It is in this region of the root that cells mature and begin to change their structure and their function. This process is known as differentiation. The epidermal cells of this zone produce long extension cells - root hairs which are a characteristic of the zone of maturation. Near the top of this zone, the cells have differentiated into the various tissues types which make up a typical root.
Beyond the region of elongation in the roots of long-lived plants, the root will start to thicken by a process called secondary thickening.
You can use diagram (left) to find a cross-section through the root where the different tissues typical of root are identified and explained.
The diagram below shows the arrangement of the tissues in a typical monocotyledonous or dicotyledonous root in which the tissues have completely differentiated. The central vascular tissues and pericycle are called the stele. (You can click on the labels for links to more information and on the stele for an enlargement of the central area)
A line diagram of a cross-section through a young root showing the location of the different tissue types.
(Click on the change button to see the difference between a monocotyledonous and dicotyledonous root)
The epidermis covers the entire root except for the root cap. Unlike the epidermis covering other plant organs, the epidermis of the root lacks a cuticle (waxy surface layer) or it is very thin and therefore does prevent the uptake of water by the root.
The cortex is formed from the ground meristem cells formed in the apical meristem of the root tip. It consists of three concentric layers of tissues.
The hypodermis In many plant species, especially those from arid areas, the hypodermis is the outermost layer/s of cells of the cortex which have suberin-enriched cell walls. These cells are only fully differentiated above the region of the root which is covered with root hairs. This layer is important in preventing the loss of water and nutrients which have been absorbed by the region of root hairs lower down the root.
The storage parenchyma cells The largest part of the cortex consists of thin-walled parenchyma cells which often have large intercellular spaces and contain starch.
The endodermis The endodermis is the layer of cells which surrounds the stele. Unlike the cortex cells to its outside, the endodermis cells are very tightly packed with no intercellular air spaces. The radial and transverse cell walls of the endodermis cells are impregnated with lignin and suberin which forms a structure known as the Casparian strip.
Two endodermis cells showing the position of the Casparian strip
If one thinks of the cells of the endodermis as the bricks which make up the wall of a tower, the casparian strips are arranged in the same way as the cement between the bricks. The Casparian strip is developed in the endodermis cell shortly behind the root tip before the region of maturation and the root hairs. It prevents water from moving into the stele through the cell walls of the endodermal cells. Water and dissolved nutrients must thus move through the cytoplasm of the endodermal cells which can therefore control the amount of water entering the vascular tissues of the central stele. The endodermis thus plays a crucial role in the uptake of water by the roots.
The diagram below shows the arrangement of the tissues in the stele of a dicotyledonous root. As can be seen in the line diagram above a monocotyledonous root differs in that it tends to have more groups of primary xylem and phloem arranged in a ring around a central parenchymatous pith (there are however, exceptions to these general rules).
A cross-section through the stele of a typical dicotyledonous root.
The stele of both monocotyledonous and dicotyledonous roots contains three main tissues-types.
The pericycle The layer of cells on the inside of the endodermis known as the pericycle. It is important as these cells are able to divide (they are meristematic). It is these cells which divide to give rise to the branch roots (also known as secondary or lateral roots). The pericyle also forms part of the vascular cambium (see below) which forms new cells when the root starts to thicken - i.e. secondary thickening.
Xylem In some plants (mainly dicots), the primary xylem (xylem which differentiated from the cells formed by the apical meristem of the root tip) forms a solid core with lobes, in which case the root is termed protostelic. In other species (mainly monocots) the vascular tissue surrounds a central parenchymatous pith and the root is siphonostelic. In siphonostelic plants the xylem forms isolated rows orientated towards the outside of the root. (Take another look at the line diagram of a cross-section showing the differences between a typical monocot and dicot root). When the xylem differentiates out of the procambium cells formed in the apical meristem of the root tip, it is the cells nearest the pericycle which differentiate first - the protoxylem and the later the cells towards the middle of the root - the metaxylem. Thus the older xylem cells are towards the outside of the root and the root is termed exarch. The opposite situation is true in a stem which is endarch.
Phloem In the root, bundles of phloem are found between the lobes of the xylem, i.e. the xylem and phloem alternate with each other. This is different to the situation in the stem where the xylem is found towards the inside and the phloem towards the outside of the vascular bundles.
The vascular cambium and secondary thickening In order for the mature root to get thicker it must produce more cells. These cells are produced by a layer of cells which are still able to divide (meristematic) - the vascular cambium. This process, called secondary thickening, takes place in the roots of dicotyledonous plants.
|A line diagram of the stele of a typical dicotyledonous root|
Click on the numbers in brackets to follow the process
of the formation of the vascular cambium on the diagram.
Cells from the pericyle, opposite the lobes of primary xylem (1) and between the primary phloem and the primary xylem (2), form the vascular cambium (3) (reset). The cells which are formed by the division of vascular cambium cells differentiate into xylem - secondary xylem which is found towards the inside of the root and secondary phloem which is found towards the outside in a similar fashion to secondary thickening in the stem.
In order to understand more about secondary thickening take a look at the section on the formation of wood on the trunk of the Ecotree (Please note if you take this link you will leave the roots and go to the stem of the Ecotree).
|Three root epidermal cells showing the three stages in the root hair development.|
Cells of the young epidermis in the region of maturation give rise to the root hairs. These are formed by extensions of the cell wall of the epidermal cells and protrude into the surrounding soil. When plants are grown in moist air, the hairs form long slender tubes but in the soil they are greatly contorted as they must grow between the soil particles. The root hairs are not separate from the epidermal cells, but form a single cells. As the cell wall extension moves out from the epidermal cell into surrounding soil, the nucleus of the cell moves to the tip of the new root hair along with much of the cytoplasm. An epidermal cell with a mature root hair thus has a large central vacuole. The cell walls of the epidermal cell and its root hair are thin, thus not inhibiting the uptake of water and dissolved salts by the epidermal cell.
Root hairs greatly increase the area of the root which is exposed to the soil and through which water and dissolved nutrients can move into the root. They are ephemeral however, and only last a few day or weeks before they wither and die. New root hairs are constantly being formed at the anterior end of the region of maturation as it is pushed forward by the growing root and those further back die. In this way the new root hairs are constantly coming into contact with fresh soil. Most plants produce roots hairs, however they are absent in certain firs and redwood species and some aquatic plants. Root hair development is suppressed when some land plants are grown with their root suspended in water (hydroponically) and their growth is negatively influenced by high soil nutrient concentrations and high and low soil temperatures.